High-Latitude Electrodynamics - The Earth's Ionosphere: Plasma Physics and Electrodynamics - page 417

yields information comparable in importance to measurements of the convective
motions.
Field-aligned currents, frequently called Birkeland currents after the man who
first postulated their existence (see Section 8.1), can be detected by the magnetic
perturbation they produce. A current flowing in a direction parallel to the earth's
magnetic field will produce a magnetic perturbation B in a direction perpen-

dicular to the field. From the diagnostic equation (2.23c) in Chapter 2, the two
quantities are related by

δ

μ 0 J

|| = ( ∇ × δ

) ||

B (8.21)

The magnetic disturbances can be observed by satellite and rocket-borne mag-
netometers that measure three mutually perpendicular components of the dis-
turbance vector. A measurement along a single trajectory does not allow the
determination of a curl, so in practice some approximations are necessary to
derive J .

Field-aligned currents frequently, but not always, occur as sheets whose dimen-
sions in one horizontal direction greatly exceed those in the other (see Fig. 8.4).
The magnetic disturbance is then predominantly parallel to the sheet, and
Fig. 8.22 shows schematically the disturbance that would be measured on passing
perpendicularly through the sheet. The current density is given by (8.21), which
simplifies to

||

, and for the dimensions shown in Fig. 8.22 the
disturbance would correspond to an outward current sheet of density 0

μ 0 J

|| = (∂/∂

x )(δ B y )

m 2 .
Most observations are not, of course, made with the convenience of such per-
pendicular crossings of the current sheet, so care must be taken in the arbitrary
application of (8.21). In general, the full perturbation vector is measured, so in
many cases only a rotation of a coordinate system is required to determine J

. 8 μ A /

.A
statistical analysis of satellite magnetometer data of this kind has enabled a sys-
tematic picture of field-aligned current directions, magnitudes, and locations to
be derived. As in the case of electric field or plasma flow observations, there exist
some relatively stable characteristics in the patterns of field-aligned currents that

||

B y

Current
sheet

100 nT x

100 km

m 2 current sheet in a plane perpendicular to the paper
and occupying the 100-km extent shown here produces a perturbation change of 100 nT
in the magnetic field.

Figure 8.22 A uniform 0 . 8 μ A /